spi: dw: Don't call kfree for memory allocated by devm_kzalloc

With devm_kzalloc, the memory is automatically freed when spi_device detach from
the bus.

Fixes: commit 43f627ac

 (spi: dw: fix memory leak on error path)
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stabe@vger.kernel.org